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The Strangeness of Charged Black Holes [Apr. 3rd, 2009|05:58 pm]
No force can resist the collapse of dense enough material into a black hole. If there is less material it may form some strange structures, like neutron stars. There may even be a further stable point, called a "quark star". But in the end, with enough mass, a black hole will form, and collapse of everything to a quantum point - the "singularity" at the centre cannot be resisted. Or perhaps not.

The description above is accurate for a "Schwarzchild" black hole, which has no spin and no charge. Although large astronomical bodies tend to be neutral in terms of electric charge, they inevitably spin. What happens inside a charged black hole is similar in some ways to what happens in a spinning one, and easier to explain. So here goes.

In a previous post I showed that pressure of a certain kind can give a negative contribution to gravity. If a black hole is made out of material which is highly electrically charged (either negative or positive - it doesn't matter), then as the material is crushed towards the centre, the repulsive pressure of the charge is very, very strong indeed, so much that it can not just resist the attractive force of gravity, but turn it around and make the centre of the black hole repulsive. Now, it would be a mistake to think of this as the electric force overcoming the gravitational force. That is not what this effect is due to. It is because the intense repulsive pressure coming from the electric field has its own gravitational effect. To put things over-simply, it isn't the electric field, it is the negative weight of the electric field. There is just so much of it. This leads to a second "horizon" within the black hole. This is where you no longer fall towards the centre. Things get very complicated at this point. I'll discuss a hugely over-simplified description, as I can show a link to an animation of this situation.

You fall within the inner horizon, and the intense repulsive gravity turns you around, and you fall back out of the black hole. But you can't. Remember that from the point of view of an outside observer, it takes you forever (or at least until the point at which the black hole evaporates) for you to fall in. Once you hit the event horizon, you have "burned your bridges". There is no way back. And yet, this repulsive force is pushing you out. Where do you go? To another universe, or to some other place and time in this universe. You will see yourself falling out of an object called a white hole; the opposite of a black hole.

Here is an animation of this:

However, this just won't happen. The "wormhole" that connects the black hole and while hole is very unstable, and can't exist in reality.

What happens at the second horizon is a subject of much current research. This is where our understanding of physics is still in progress. Can you safely pass through? If so, into what? What happens then?

Finally, I'll mention why charged black holes (which are extremely unlikely to exist) are similar in a way to normal, spinning black holes. In the charged black hole the repulsive gravity was a result of a highly compressed electrostatic field. In a spinning black hole, there is repulsive gravity as a result of the momentum of the spinning compressed mass. So, there is also a second horizon at which the inevitable drift to the centre ceases. What is within that second horizon is something very strange. Not a point, but a ring "singularity". What happens if you pass through that ring? No-one knows.

It is time to review how we got here. There have been virtually no equations. It has all been from some very simple ideas of symmetry. There is a limiting velocity, which we saw from electromagnetism. That gave the distortions of space and time of special relativity. Those distortions showed that mass and energy were interchangeable. That we can't know if we are accelerating or resisting gravity (because they are the same thing), gave us spacial distortions, and frame dragging (a sort of gravitational equivalent of magnetism). The equivalence of mass and energy meant that we have to take pressure into account to predict the curve of spacetime. Negative pressure means a repulsive gravity.

There is no doubt that Einstein was one of the greatest scientific geniuses who ever lived. He demolished the inconsistencies and assumptions of physics to show that the universe is an incredibly simple place, with much of what we think of as physical law being a result of our parochial observations, nothing more. But his simple vision leads to vistas beyond anything we could imagine.

There is much left to simplify in our understanding of the universe. Quantum Mechanics and General Relativity remain fundamentally different and incompatible views of reality. Sometime soon, I'll write about attempts to bring them together.